1. Field of the Invention
The present invention relates to a white balance adjuster of an electronic still camera for recording a still image.
2. Related Background Art
A human eye has an adaptability that a white object appears white even if an illumination light varies. On the other hand, in a video camera or an electronic still camera which converts an image to an electrical signal for recording, a color temperature of an illumination light around an object is metered and a white balance is adjusted for the image signal based on the color temperature so that a white object appears (is recorded or reproduced) white.
A configuration of a white balance adjuster in a prior art color camera is shown in FIG. 3. In FIG. 3, a light L from an object is sensed by an image pickup unit 53 through an optical member 51 and a diaphragm member 52. The image pickup unit 53 converts an input light to an electrical signal and comprises a solid-state image pickup device such as a CCD (charge transfer device) or a MOS device and a circuit for sampling an output signal from the image pickup device at a predetermined timing, and produces chrominance signals for R (red), G (green) and B (blue).
The G output from the image pickup unit 53 is supplied directly to the signal processing unit 55 but the R output and the B output are supplied to the signal processing unit 55 through white balancing variable gain amplifiers 54R and 54B which are controlled by a control unit 50 is accordance with an output from a color metering unit 58. The signal processing unit 55 is connected to a recording unit 56 which adjusts gains for the chrominance signals, set-up levels, gammas and knees, and records an image signal. The diaphragm member 52 is controlled by the control unit 50 in accordance with the output from a photometering unit 57 which meters a brightness of the object.
The control of the white balancing variable gain amplifiers 54R and 54B is now explained. In the color metering unit 58, the light around the camera is directed to R photo-sensor 72R, a G photo-sensor 72G and a B photo-sensor 72B through a diffusion plate 71, and it produces a red component (Rb), a green component (Gb) and a blue component (Bb) of the incident light. Those color component signals are logarithmically compressed by a logarithmic compression circuit 73 which produces outputs log Rb, log Gb and log Bb. They are converted to log (Rb/Gb) and log (Bb/Gb) by subtractors 74 and 75, and reverse-logarithmically converted to Rb/Gb and Bb/Gb by an expander 76. Then, they are supplied to an A/D converter 59 through a current-voltage converter 77, and digital outputs thereof are supplied to the control unit 50. The gains of the variable gain amplifiers 54R and 54B are controlled in accordance with the digital outputs of Rb/Gb and Bb/Gb detected by the color metering unit 58 by referring a white balancing control Table 61.
As shown in FIG. 4, the diffusion plate 71 through which the light is directed to the color metering unit 58 is usually arranged at a front top of the camera while facing front (object) to sense a camera peripheral light Lp, and the photometering unit 57 usually meters the light directed through the optical member 51 by splitting it in the camera.
In the prior art camera, a color temperature around the object and a color temperature in the vicinity of the color metering device may be detected differently in a fine weather photographing condition. For example, when an object is to be photographed in a forward light condition, the vicinity of the color metering device is shaded by the camera or a photographer because a light incident area to the color metering device is usually on the front plane of the camera, and hence the color temperature of the periphery of the object is different from the color temperature of the vicinity of the color metering device. If the white balance is adjusted based on the output of the color metering device, the object is photographed reddish.
It is ideal that the white balance control table is changed in accordance with the condition of the illumination light to the object. For example, since the change in the color temperature varies with a condition of fine weather at out-door, cloudy weather at out-door or in-door, it is desirable to use different white balance control tables. However, in the prior art adjuster, a white balance control table determined by a rule of greatest common divisor is used to adjust the white balance and proper white balance adjustment to cope with the condition of the illumination light to the object is not attained.